A brushless direct current (BLDC) motor is a type of permanent magnet synchronous motor powered by direct current, as its name suggests. The BLDC motor generally includes a moving rotor with permanent magnets and a stationary stator that includes windings of wire into which a voltage is induced as the magnets pass over them. One common configuration of BLDC motor is the three-phase configuration in which the stator has three windings.
The BLDC motor generally operates by electronic commutation in which the BLDC motor is driven according to a commutation sequence. In a three-phase BLDC motor, for example, each step of the commutation sequence includes one winding energized positive (current entering) and another winding energized negative (current exiting), while the third winding is not used (non-energized). Driving two of the three windings generates a magnetic field in the stator windings and the rotor magnets, which produces torque that causes the rotor to rotate. To keep the rotor rotating, the sequence moves to a next step for another winding pair to shift the position of the magnetic field produced by the windings. For a three-phase BLDC motor including phases A, B and C each of which is separated by adjacent phases by 120°, one example commutation sequence may include the following six steps AB-AC-BC-BA-CA-CB.
Many BLDC motors are driven by an inverter (e.g., three-phase inverter) and require the position of the rotor to apply the proper commutation sequence. The rotor position may be obtained in a number of different manners. In one common manner, the BLDC motor includes a position sensor, such as a Hall-effect sensor, that directly senses the position of the rotor. Other BLDC motors operate by sensorless control in which the position of the rotor is estimated using other motor parameters. In one common sensorless control technique, the position of the rotor is estimated using back electromotive force (EMF) generated in the non-energized winding during operation.
Whereas the back-EMF measurement technique is adequate for estimating the position of the rotor, the technique has it limits. The magnitude of the back-EMF generated in the non-energized winding is proportional to the speed of the motor. In a number of instances in which the motor is stationary or rotating at low speed, the quality of the back-EMF measurement (e.g., signal-to-noise ratio) may be too low to effectively estimate the position of the rotor.